The invention relates to the preparation of alkoxymethylenemalonic acid esters by the reaction of malonic acid esters with orthoformic acid esters.
It is known that the alkoxymethylenemalonic esters are formed by the action of orthoformic acid esters on malonic acid dialkyl esters with the yielding of alcohol. The condensing agents used L. Claisen (Berichte 26, 2729 (1893), Ann. 297, 19 (1897) were acetic acid anhydride and anhydrous zinc chloride as catalysts. The reaction mechanism of this condensation was thoroughly studied by Post (J. Org. Chem. 2, 260 (1937) and Fuson (J. Org. Chem. 11, 194-8 (1946).
In the above-cited publications, approximately 2 moles of acetic acid anhydride were used per each mole of orthoester and 0.8 to 1.35 moles of malonic ester.
According to these publications, the best yields of ethoxymethylenemalonic acid diethyl ester (EMME), were 78.8% of the theory with respect to malonic acid diethyl ester, and 46% of the theory with respect to orthoformic acid ester.
Furthermore, U.S. Pat. No. 2,824,121 describes, among other things, the preparation of EMME by the reaction of malonic acid diethyl ester with orthoformic acid triethyl ester in the presence of acetic acid as the sole catalyst. It states that alkoxymethylenemalonic acid esters can be obtained with a yield of 91.8% of the theory with respect to malonic acid ester and 73.5% of the theory with respect to orthoformic acid ester, but with a transformation of only 60% of the starting substances, which it has not been possible to confirm by tests.
These known methods have considerable disadvantages. The reaction of the two components with one another is very incomplete and/or numerous by-products are formed which diminish the yield considerably. The latter are either volatile, but separable only with difficulty from EMME by distillation, such as the dialkoxymethylmalonates, for example, or they are solid, such as the pyrone derivatives which form by condensation and cyclization. This makes it very difficult in the one case and impossible in the other to produce and isolate the pure alkoxymethylenemalonic esters economically.